Twenty-five years after the sudden death of her 16-year-old daughter Pauline, Kathy Moyle was finally told it was the result of a genetic syndrome and that she, too, had it. By then, sudden death syndrome had almost certainly taken the lives of her other daughter, Lynn, at the age of 38, and her grandson Matthew at 17.

Pauline was cradling her eight-week-old baby girl in her arms when, in Kathy's words, she "dropped dead in front of my eyes". "She was such a strong, healthy girl," says Kathy, now 67, from Devon. "I was in a state of utter shock and disbelief."

However, when the coroner's report stated that Pauline had an abnormality in a coronary artery and her death was due to natural causes, Kathy accepted it. "With three other children to look after as well as Pauline's baby, I didn't have the time or the energy to fight to find out more – something I feel very guilty about to this day," she says.

But in 2001 Matthew, an RAF cadet, was riding his bicycle when he too collapsed and died. The family could not believe there had been another sudden death. Again, there was no inquest and the coroner's report stated that the death was due to epilepsy. No one suggested to Kathy that there might be a hereditary link, or that members of her family should be screened for heart defects.

A year later, Lynn was watching television one evening when she fell to the floor and died. Finally, Kathy says, the Exeter coroner's office asked for permission to send Lynn's heart to a specialist for examination. However, instead of being dispatched by special courier, the heart was sent off by post and it never reached its destination, something the coroner's office revealed to Kathy six months later. At the inquest into Lynn's death, the coroner recorded an open verdict.

"I was so upset and angry. I couldn't believe that after we'd granted permission for them to remove Lynn's heart, they chose to send it off in such a cheap fashion. Removing her heart turned out to be a complete waste of time. The vital clues it held that could have helped the rest of us were lost. We felt so terribly let down," she says. The current Exeter coroner, Dr Elizabeth Earland, was not in the job at the time of the Moyle family inquests and does not wish to comment on the case.

It was not until she was watching a TV documentary that Kathy heard about Long QT syndrome, a rare inherited disorder that affects the electrical rhythm of the heart. After speaking to a geneticist at the Royal Devon and Exeter hospital about her fears, the family were referred to a specialist at St George's hospital in London for screening.

It was there that Kathy was finally told that it was believed her daughters and grandson had died as a result of having the same fatal genetic disorder, and that she herself had the condition.

Explaining the case, Dr Sanjay Sharma, professor of clinical cardiology at St George's, says: "In the context of a personal history of Long QT syndrome, I have no doubt that Kathy's two daughters and her grandson died as a consequence of the disorder. Long QT syndrome is inherited as an autosomal dominant trait – in other words, there is a one in two chance of inheriting the disorder if a parent is affected."

For Kathy, the news was devastating. "If Matthew and Lynn had been given an ECG screening after Pauline's death, they might still be here," she says.

Such cases are not unusual, with footballer Fabrice Muamba's recent cardiac arrest on the pitch drawing attention to potentially fatal heart problems among the young. Heart charity Cardiac Risk in the Young (Cry) is campaigning to make coroners more accountable and to ensure that in cases of sudden death the hearts of the young victims are sent to cardiac pathologists for expert examination.

Cry's chief executive, Alison Cox, says the lack of follow-up action after a sudden death is a widespread problem. "What happened to Kathy's family beggars belief, but we don't think hers is an isolated case."

According to the charity, 12 apparently healthy under-35s die suddenly every week in the UK. (The charity estimates the numbers because not all sudden deaths are recorded as such). In at least 20% of cases, Cry claims, coroners fail to fast-track hearts to an expert cardiac pathologist. This would establish whether the young person had died from an inherited cardiac condition and would help to prevent further deaths in the family through screening and treatment. Coroners' inaction means that inherited, but often preventable, heart conditions in other family members are missed and bereaved families have gone on to suffer further sudden deaths.

Cox says: "We've got a postcode lottery. Some families report appalling treatment. The problem is that coroners aren't accountable and there is no one you can complain to about poor performance. We still have too many coroners who don't make a proper effort to investigate the cause of death when a hitherto healthy, fit young person dies suddenly."

Cry hopes the justice secretary Ken Clarke's proposed appointment of a chief coroner will help to drive up standards and prevent families such as Kathy's suffering so many young deaths.

"Having a condition like this is like living with Russian roulette," Kathy says. "You wonder who'll be next. I'm desperately worried for my other five grandchildren and three great-grandchildren."